New Delhi: An unprecedented dense fog on Friday left flight operations in disarray at IGI airport, affecting 400 flights and giving a harrowing time to flyers.

The fog enveloped the airport around 4.00 AM and general visibility reached near zero while the runway visibility dipped to less than 50 metres leading to suspension of operations for seven hours.

"The situation prevailed till 11.00 AM, during which almost all flight operations were affected as the runway visual range Runway 29 (new) and 28 (main) were lower than the prescribed minima for flying in CAT III B conditions (below 50 metres)," an airport official said.

According to rules, the flights should take off when the runway visibility is more than 125 metres and 150 metres for different types of aircraft.

Though the two runways of Delhi airport are CAT III B compliant, under this an aircraft can land when the runway visibility is not less than 50 metres, but the runway visibility went below 50 metres.

The poor visibility forced the airport authorities to implement low visibility procedures on new runway at 2125 hours on Thursday and was in progress till late this evening.

The LVP was initiated at main runway (Rwy 10/28) at 0112 hours and was also in progress till evening.

About 25 flights -- 23 domestic and two international -- departing from Delhi and 19 flights (2 international and 17 domestic) arriving here were cancelled.

Poor visibility forced the airport authorities to divert 30 (19 domestic and 11 international) flights from here to nearby airports.

On Thursday, around 143 flights were delayed and nine cancelled due to poor weather at destination or operational reasons.